US researchers have grown tree-like networks of branched
nanowires, which they speculate might lead to "three-dimensionally interconnected
computing structures analogous to the brain".

Charles Lieber and colleagues at Harvard University
in Cambridge, Massachusetts, have found a way to make nanoscale wires
made from semiconductors sprout branches. By repeating the branching
procedure, they have grown secondary branches from the first ones, creating
semiconducting nano-trees1.

By growing each generation of branches from a different
type of semiconductor — for example, from p-type and n-type materials,
in which an electrical current is carried respectively by positively
and negatively charged particles (holes and electrons) — the researchers
can turn the branching points into electronic devices. Junctions between
p-type and n-type semiconductors can act as diodes, for example.

In this way, the branched nanowire structure becomes
a collection of devices wired together, which might function as a circuit.
But to build this in a fully rational way, the researchers will have
to gain more control over the sprouting of new branches.

Lieber's group has previously developed methods for
making nanowires from various semiconducting materials: silicon, so-called
III-V semiconductors such as indium phosphide, and II-VI semiconductors
like cadmium selenide and zinc sulphide. The technique involves chemical
deposition of a vapour of the semiconducting material onto catalytic
metal nanocrystals. The diameter of the wires is controlled by the diameter
of the catalyst particles.

To make branched nanowires, this process is simply repeated
with new catalyst particles attached to an existing wire. Lieber and
colleagues have demonstrated the principle using silicon nanowires catalysed
by gold, and gallium nitride wires catalysed by nickel. They deposit
the catalytic nanoparticles from solution onto an existing 'backbone'
wire, using particles that are somewhat smaller in diameter than the
wire itself.

For example, gold nanoparticles 20 nm wide, distributed
sparsely along the length of a 30-nm silicon nanowire, generated branches
about 22 nm wide. The density of these branches is determined by the
concentration of catalyst particles in the original solution. As yet,
however, the researchers don't have any detailed control over where
the branches sprout.

On silicon, new branches grow at fairly well-defined
angles of 60°–70° relative to the backbone. By inspecting the junctions
closely in a transmission electron microscope, Lieber and colleagues
saw that the growth is epitaxial: the atomic lattice of the new branches
is aligned with that of the backbone. This means that the branching
angle is determined by the crystallographic axes of the material.

That's important, because it means that if the backbone
and branches are, say, p-type and n-type silicon respectively, there
is a smooth, high-quality junction between the two that can display
device-like electronic characteristics. The Harvard group's preliminary
studies show that indeed such junctions can act not only as p–n diodes
but also as field-effect transistors. In other words, the branching
nanostructure can contain electronic switches linked by semiconductor
wires.

So far, the researchers have got as far as making second-generation
branches, growing 10-nm wires on the 20-nm branches from a 40-nm silicon
backbone by using catalyst particles of the respective diameters. The
hierarchy of branches is not yet perfect, however: some 10-nm gold nanoparticles
stick to the backbone as well as to the 20-nm branches, causing 10-nm
wires to sprout directly from the backbone. The deposition of the catalyst
particles must somehow be made more selective if this is to be avoided.
And to link the branches up into a complex network like the vascular
or neural networks of biology, it will be necessary to find a way of
getting the branches on separate backbones to fuse together.